Many electronic products implement certain functions which are required to be synchronized with the AC line voltage which powers the product. One such product is an oscilloscope, and the function referred to is AC Line Triggering.
It is necessary to derive an AC signal from the AC power line and convey it to a processing circuit in order to implement the desired function. A transformer is often used for this purpose. The transformer is insensitive as to whether the AC voltage is applied to either side or to both sides of the two-wire AC power system. The transformer provides the required safety isolation from the AC line, and produces a signal which is a replica of the AC line voltage. Unfortunately, because the transformer and associated circuitry are coupled to the AC power line, the transformer must be certified by certain regulatory agencies, and any changes in the design will cause the need for a recertification. A transformer also undesirably involves expense in its initial cost and handling.
Another device which is often used to derive a signal from the AC line and convey it to a processing circuit, is an optical isolator. An optical isolator utilizes a current limited LED on the input side, and a phototransistor on the output side. The coupling between the input and out sides of the optical isolator is by means of the light generated by the LED in response to the AC line signal, which illuminates the base portion of the phototransistor, causing it to conduct. The optical isolator shares the same disadvantages as the transformer in terms of the need for proper testing and certification, and in terms of its cost. Additionally, optical isolator circuits exhibit undesirable trigger timing variations in response to amplitude variations in the AC line voltage.
What is needed is an inexpensive way to derive a signal from the AC line, which requires no testing and certification by an agency, and still provides the required safety isolation.